This invention is concerned with the design of liquid crystal displays and particularly with techniques for maximizing the field of view of such displays by maintaining a high contrast ratio at wide viewing angles.
Liquid crystals are useful for electronic displays because polarized light traveling through a thin film of liquid crystal is affected by the birefringence of the film and the birefringence can be controlled by the application of a voltage across the film. Liquid crystal displays became desirable because the transmission or reflection of light from an external source, including ambient light, could be controlled using a liquid crystal device with much less power than was required for the luminescent materials used in other displays. As a result, liquid crystal displays are now commonly used in such applications as digital watches, calculators, portable computers, and many other types of electronic equipment, exhibiting in these applications the advantages of very long life and operation with very low weight and low power consumption.
The information in many liquid crystal displays is presented in the form of a row of numerals or characters, which are generated by a number of segmented electrodes arranged in a pattern. The segments are connected by individual leads to driving electronics, which applies a voltage to the appropriate combination of segments to display the desired information by controlling the light transmitted through the segments. Graphic information or television displays may be achieved by a matrix of pixels which are connected by an X-Y sequential addressing scheme between two sets of perpendicular conductors. More advanced addressing schemes use arrays of thin film transistors to control the drive voltage at the individual pixels. The latter scheme is applied predominantly to twisted nematic liquid crystal displays.
Contrast is one of the most important attributes determining the quality of a liquid crystal display. The primary factor limiting the contrast achievable in a liquid crystal display is the amount of light which leaks through the display in the dark state. This problem is exacerbated in a high ambient light environment, such as direct sunlight, where there is a considerable amount of reflected and scattered ambient light. In addition, the contrast ratio of the liquid crystal device also depends on the viewing angle. Contrast ratio in a typical liquid crystal display is a maximum only within a narrow viewing angle centered about normal incidence and drops off as the angle of view is increased. This loss of contrast ratio is caused by light leaking through the black state pixel elements at large viewing angles. In color liquid crystal displays, such leakage also causes severe color shifts for both saturated and gray scale colors. These limitations are particularly important for applications requiring a very high quality display, such as avionics, where viewing of avionics displays from both pilot and copilot seating positions is important. It would be a significant improvement in the art to provide a liquid crystal display capable of presenting a high quality, high contrast image over a wide field of view.
Yeh, et al., "Compensator for Liquid Crystal Display . . . ", U.S. Pat. No. 5,196,953, discloses a novel structure for a compensator in such a liquid crystal display, making possible a considerable improvement in the viewing contrast and color rendition of such displays at oblique viewing angles. Further improvements in contrast and viewing angle, however, are desirable for such displays.